The demand for reliable and high data throughput wireless communication networks has never been as great as in the present. While initial consumer and business demand was for wireless communication technologies to support voice communication, this demand has grown both in terms of the sheer volume of users as well as the bandwidth requirements; the latter being the result of demand for wireless broadband data services. These services are provided, for example, by Fourth Generation (“4G”) wireless systems based on 3GPP Long Term Evolution (“LTE”), IEEE 802.16e WiMax, and 3GPP2 Ultra Mobile Broadband (“UMB”), each of which use orthogonal frequency division multiple access (“OFDMA”) technology as the air interface technology.
Considering LTE in particular, the main goals include providing peak data rates up to 100 Mbps in the downlink and up to 50 Mbps in the uplink, reduced latency, significantly improved spectrum efficiency, improved system capacity, coverage, and reasonable system & terminal complexity. In order to achieve these goals, several new radio transmission technologies have been proposed. The LTE downlink uses orthogonal frequency division multiplexing (“OFDM”) as an accessing technology, while the LTE uplink uses single carrier frequency division multiple access (“SC-FDMA”). Multiple antennas at the transmitter and receiver increase the data rates as well as achieve diversity gains. By transmitting multiple parallel data streams to single terminal, data rates can be increased significantly. On the other hand, multiple input multiple output (“MIMO”) systems are used for increasing the diversity by transmitting the same symbol on different antennas. Furthermore, as in 3G systems, Adaptive Modulation and Coding (“AMC”) is used in LTE to exploit channel information.
In practice, link adaptation (“LA”) in AMC may fail due to inaccuracies in link estimation and the feedback delays in channel quality measurements. To recover from link adaptation errors, hybrid automatic repeat request (“HARQ”) is typically used as a retransmission mechanism. HARQ is used in wireless systems to overcome transmission errors that cannot be corrected using forward error correction (“FEC”). HARQ improves the decoding probability by using information from previous transmissions. Depending on the way the retransmission packets are combined, HARQ systems can be typically classified into two categories namely, Chase combining (“CC”) or incremental redundancy (“IR”).
In CC, the basic idea is to send a number of repeats of each coded data packet and allowing the decoder to combine multiple received copies of the packet before decoding. The retransmitted packet is an exact replica of the original packet. In this way, the time diversity gain can be realized. This scheme requires less implementation complexity at the receiver.
In IR, instead of sending simple repeats of the entire packet, additional parity information is incrementally transmitted if the decoding fails in the first attempt. Each transmission may or may not be self decodable. If each transmission is self decodable, then it is called partial IR, otherwise full IR.
Link adaptation decisions for a mobile communication device, i.e., mobile station (“MS”) or base station (“BS”), are based on a reference signal (“RS”) signal to interference ratio (“SINR”) estimate. Conventional link adaptation estimates the SINR from the reference signal measurements and determines the highest MCS that can be supported at a pre-determined FER using 1 HARQ transmission.
Link level performance curves show significant signal to noise ratio (“SNR”) gains when using HARQ-IR over HARQ-CC due to the transmission of additional parity information in the former. However, conventional LA schemes are unable to take advantage of these HARQ gains. In a multi-user environment, conventional link adaptation does not provide any spectral efficiency gains using HARQ_IR relative to using HARQ-CC.
Therefore, what is needed is a method and apparatus for improving the overall system performance and spectral efficiency of an OFDMA system using link adaptation schemes that can extract gains offered by HARQ.